1. Field of the Invention
The present invention provides a tandem axle suspension of the type disclosed in U.S. Pat. No. 4,741,553, for use with trailer tag axles with structural modifications being made thereto adapting the tandem axle suspension of the present invention for use with a vehicle having a tandem pair of drive axles.
2. Description of the Related Art
Tandem axle suspension systems are commonly employed with trailers having one or more trailing axles. An example of such a prior art tandem axle suspension system is disclosed in U.S. Pat. No. 4,741,553, incorporated herein by reference. This prior art suspension is comprised of an air spring suspension assembly combined with a leaf spring suspension assembly.
The leaf spring supports the trailer load on the forward axle of the trailer's tandem axles. The forward end of the leaf spring suspension assembly is mounted to a forward hanger secured to the trailer chassis. The rearward end of the leaf spring assembly is mounted to a forward end of a bolster beam that is itself pivotally mounted to a rearward hanger assembly. The rearward hanger is also secured to the chassis. A radius rod is typically connected from beneath the forward axle to the forward hanger to be pulled in tension and resist torsional forces exerted on the forward axle when the trailer is pulled.
The rearward hanger assembly also supports a torque beam mounted to the hanger assembly by a single bushing. The rearward axle of the trailer is mounted to the torque beam by two more bushings, and an air spring is connected between the torque beam and a rearward end of the bolster beam. This prior art tandem axle suspension system is specifically designed to support loads exerted on the system when the trailer is pulled and both the forward and rearward axles of the tandem are tag axles or provide no driving force. It should be appreciated that such a suspension system would not be adequate for use with a vehicle having tandem drive axles. Where both the forward and rearward axles of a tandem are powered by a vehicle's engine the torsional forces exerted on these axles when the vehicle first moves in driving operations are very different from those exerted on the tandem axles of a trailer where no driving torque is delivered to the axles. For example, with the forward axle being a drive axle the torsional forces exerted on the axle at the initiation of driving operations of the vehicle are opposite to those torsional forces exerted on a forward tag axle of a trailer being pulled. Therefore, the radius rod of the prior art tandem axle suspension systems that extends forward from beneath the front tag axle to a forward hanger to resist torsional forces exerted on a forward tag axle of a trailer would be insufficient to resist torsional forces exerted on a forward drive axle of a tandem axle vehicle. Moreover, the pair of bushings mounting the rear tag axle of a trailer to a torque beam could not withstand the torsional forces exerted on a similarly mounted drive axle over a period of use.